The invention relates to a method for controlling a pneumatic brake system of a utility vehicle having a service brake and an electric parking brake, wherein an electronic control unit is provided for controlling the electric parking brake.
Such devices and methods are becoming increasingly important in the context of operating utility vehicles. In terms of the further development of the systems, emphasis is placed on various aspects, in particular operating comfort and safety.
Since safety aspects play a decisive role in particular in utility vehicles, emphasis has been placed on efforts to improve safety. These efforts relate in particular to the brake system, and in the event of a defect in one of the brake circuits it is necessary to ensure that the vehicle nevertheless comes to a standstill safely. However, if one of the service brake circuits fails it is firstly unavoidable that braking operations occur with a reduced overall braking force. The possibility of hazardous situations occurring is therefore not ruled out.
The invention is based on the object of improving the safety of the brake system of a utility vehicle with respect to the possible failure of a service brake circuit.
The invention is also based on the object of improving the operating comfort of an electric parking brake.
The invention builds on the prior art in that, in the case of a circuit defect in the service brake, braking of the utility vehicle is assisted by the parking brake. Since the electric parking brake is actuated by an electronic control unit which is integrated into the control system of the utility vehicle, and is in particular connected to the CAN bus, it is possible for the electronic brake controller to issue a request to the electronic control unit of the parking brake, and the parking brake then assists the braking operation. Consequently, even when there is a circuit defect, it is possible to bring the vehicle to a standstill with a comparatively high braking force, and at any rate the braking force here is higher than if the braking force of a single service brake circuit were relied on.
The braking assistance is provided by the parking brake by virtue of the fact that the spring-loaded cylinders of the parking brake are vented in accordance with a predefined time/pressure characteristic curve. This makes it possible to apply the parking brake gently. This is advantageous since sudden venting of the spring-loaded cylinders should be carried out only at extremely low speeds of the utility vehicle. Rapid venting of the spring-loaded cylinders at high speed would very probably lead to an accident. However, if a time/pressure characteristic curve, which is sufficiently flat is prescribed, the parking brake can be used to provide assistance even at relatively high speeds.
In this context. it is useful that different time/pressure characteristic curves are used as a function of the speed of the utility vehicle.
The invention is also advantageously developed in that different time/pressure characteristic curves are used as a function of the faulty brake circuit of the utility vehicle. In the event of a failure of a brake circuit, the braking effect of an axle of the utility vehicle, which is assigned to this brake circuit, does not occur. Depending on whether the axle is a front axle or a rear axle, different assisting braking effects of the parking brake are desired.
According to a further embodiment of the present invention, further conditions have to be met for the parking brake to close, specifically a switched on ignition and a sufficiently low speed of the utility vehicle. Even if assistance is to be provided even at a high speed, specifically through the use of a suitable time/pressure characteristic curve, checking for a sufficiently low speed is a measure which improves safety.
In this context, it is useful that the presence of a sufficiently low speed of the utility vehicle is determined by comparing the wheel speed, the vehicle speed output of a controller and the rotational speed of the transmission with predefined threshold values. The wheel speed can be determined directly by way of a sensor, the vehicle speed is generally available as an output value of a controller and it is generally input into the CAN bus, and the rotational speed of the transmission is supplied by the electronic transmission controller and is preferably superimposed on the CAN bus.